The present invention relates to radio frequency antennas or more specifically to quadrifilar helix antennas.
A quadrifilar helix antenna typically consists of four symmetrically positioned helix shaped metallic wire of strip elements. The four helices are fed in phase quadrature, i.e. with equal amplitude and with the phase relation 0xc2x0, 90xc2x0, 180xc2x0 and 270xc2x0. The quadrifilar helix antenna can receive and transmit circular polarised signals over a large angular region. Its radiation characteristics is determined mainly by the shape of the helices, i.e. the number of turns, pitch angle, antenna height and antenna diameter, and in the case of conical shaped helices also the cone angle.
The phase quadrature feeding of the four helices can be accomplished in different manners. One possibility is to have a separate feeding network that generates the phase quadrature. Alternatively a balun system can be used combined with a separate 90xc2x0-hybrid or with a self-phasing helix antenna.
A difficulty with the traditional quadrifilar helix antenna is its relatively strong frequency dependent input impedance. This makes it difficult to design broad band matched or dual-frequency matched antennas. However, this problem can be solved to some extent by having a double tuned quadrifilar helix antenna.
Dual frequency quadrifilar helix antennas are frequently requested for many applications commonly for the purpose of having separate frequency bands for receiving signals and for transmitting signals.
For mobile satellite communication system, dual-frequency circularly polarised antennas are requested for the use on hand held terminals. These antennas are designed to operate at L- or S-band with a coverage over a cone with a half angle between 40xc2x0 up to 90xc2x0 depending on the system.
One object of the invention is to provide a novel compact dual-frequency quadrifilar helix design that has the potential of low cost mass production A second object is to provide a dual-frequency quadrifilar helix antenna design that makes a simple mechanical design possible and suitable for space applications.
The present invention is a mechanically simple dual-frequency (or wide band) quadrifilar helix antenna. It includes four helix shaped radiating elements where each helix element consists of two or more parallel helices of different lengths that are in galvanic contact at, or close to, the feeding point. The four feeding points of the helix elements are located at the bottom of the helix, meaning that the feedings of the helix elements are located at the end of the helix pointing in the direction opposite to the direction of its main radiation.
The present invention also includes a compact dual-frequency (or wide band) quadrifilar design with an integrated feeding network (power distribution network). In this case the four feeding points of the helix elements are connected via small matching sections to a distributed series feeding network consisting of transmission lines that serves for the phase quadrature feeding of the four helix elements, yielding a single input feeding point for the complete antenna assembly. The matching section and the series feeding network is preferably realised in stripline or microstrip techniques.
By providing a quadrifilar helix antenna of the suggested design it becomes a very attractive candidate for use in mobile satellite communication systems as an example, but it requires a compact dual-frequency design with an integrated feeding network that is simple from a manufacturing point of view.
Further, in mobile satellite communication systems a dual-frequency design is very attractive as it is simple from a manufacturing point of view. Very often a simple mechanical design means a safe design for space applications.
Quadrifilar helix antennas can also be used in applications as transmission and/or receiving antennas on board satellites.